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Technologic Papers 



OF THE 



Bureau of Standards 

Sv W. STRATTON. Director 



No. 139 

SOME TESTS OF LIGHT ALUMINUM CAST- 
ING ALLOYS-THE EFFECT OF 
HEAT TREATMENT 



BY 



P. D. MERICA, Physicist 



and 



C. p. ZARR, Associate- Physicist 

Bureau 0/ Standards 



ISSUED OCTOBER 24, 1919 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents. Government PrlnUng Offic* 
Woshington, D. C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1919 



Monograph 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 139 

SOME TESTS OF LIGHT ALUMINUM CAST- 
ING ALLOYS-THE EFFECT OF 
HEAT TREATMENT 



BY 

P. D. MERICA, Physicist 

and 

C. p. KARR, Associate Physicist 
Bureau of Standards 



ISSUED OCTOBER 24, 1919 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1919 







-r^V,^ 



OEG $ 1919 



1^ 



■►^ 






SOME TESTS OF LIGHT ALUMINUM CASTING AL- 
LOYS—THE EFFECT OF HEAT TREATMENT 



By P. D. Merica and C. P. Kan 



CONTENTS 

Page 

I. Introduction 3 

II. Preparation of the alloys 4 

III. Methodsof test 4 

IV. Influence of form of test specimen 5 

V. Influence of melting and pouring temperatures 8 

VI. Influence of chemical composition g 

VII. Effect of heat treatment 11 

VIII. Relation between tensile properties and hardness 12 

IX. Microstructure 13 

X. Resistance to corrosion in the salt-spray test 14 

XI. Resistance to the action of alternating stresses 15 

XII. Summary and conclusions 18 

I. INTRODUCTION 

Dtiring the past 18 months a number of experimental heats of 
different compositions of Hght alimiinimi casting alloys have 
been poured at the Bureau of Standards foundry, and mechanical 
and corrosion tests made of the test-bar castings. These tests 
have not been of a particularly systematic nature, but have been 
directed at different times toward different objectives, such as (i) 
the development of a suitable ductile casting alloy, (2) the deter- 
mination of the resistance of standard alloys to alternating 
stresses, (3) the effect of heat treatment on the mechanical prop- 
erties of casting alloys, and (4) the study of the effect of type of 
test bar. Much of the work was done in connection with the 
activities of the subcommittee on aluminum of committee B-2 of 
the American Society for Testing Materials on nonferrous metals 
and alloys. The grouping of the results of these tests and dis- 
cussion of them within the scope of one paper is somewhat 
arbitrary, but constitutes perhaps the best presentation of some 
of the facts which have developed during the coiurse of the 
investigations. 

3 



4 Technologic Papers of the Bureau of Standards 

Many of the questions studied have received attention at the 
hands of previous investigators; a summary of all of this work, 
together with a bibliography of it, is contained in a recent 
publication.^ 

11. PREPARATION OF THE ALLOYS 

The alloys were all prepared with commercially pure materials: 
99 per cent aluminum; Commercially pure copper; Thermit 
manganese; Commercially pure nickel; Coromercially pure spelter; 
magnesiimi, 99.9 per cent pure. Hardeners, prepared in a crucible, 
were used for the introduction of these elements, as follows: (a) 50 
per cent copper, 50 per cent aluminum ; {h) 50 per cent aluminum, 
25 per cent copper, 25 per cent manganese; (c) 80 per cent 
aluminum, 20 per cent nickel. In preparing a heat, the required 
amount of hardener together with the aluminiun was placed in 
the crucible and melted; a No. 40 plmnbago crucible was generally 
used. The metal was stirred with a graphite rod, the pot removed 
from the furnace, and the magnesium added and submerged 
within the melt with tongs. Rock salt was added as a flux 
during the melting, and just before pouring an otmce or so of zinc 
chloride was thrown on the metal to bring up and remove the 
dross. 

The earlier heats, viz. Hi, H2, E5, E6, By, A2, A4, E8, E-C 
and D, Gi, G2, C15, C16, C17, C18, C19, A30, and ^31, were 
melted in an oil-fired ftunace, the others in a gas-fired one. The 
highest temperature dining melting and the pouring temperature 
were measirred with a thermocouple protected with iron pipe. 

All specimens, except those of type //, were poured in green 
sand, skin dried with the torch; those of type // were poured in 
core sand. 

Heats X-i and 2, XE-3 and 4, C56, XE-5 and 6, XE-7 and 8, 
XE-9 and 10, C20, C57, C49, C59, C53, E21, E22, H5, H6, ZiA, 
E9, G7, G8, ^15, Z16, G9 and Gio were melted in a covered 
crucible, the others without cover. 

III. METHODS OF TEST 

All test bars for the tensile test were machined to 0.505 inch 
diameter over the gauge length of 2.25 inches. Types /, //, ///, 
and VII (see pages 6 and 7 and Figs, i and 2) were tested with 

'Aluminum and its Light Alloys, Circular No. 76 of the Bureau of Standards, 1919; also in Chem. and 
Met. Engineering, 1513. 



Tests of Light Casting Alloys 5 

wedge grips. Type IV had threaded ends, types V and VI had 
shoulders with which the specimens were held in self-centering 
grips. • 

The yield point and proportional limit were determined with 
the Riehle improved extensometer ; the yield point was taken as 
that value of the stress at which the slope of the stress-strain curve 
was twice that of the elastic portion of it. 

The Brinell hardness, using a ball of 10 mm diameter, a load 
of 500 kg applied for 30 seconds, and the sceleroscope hardness 
with magnifying hammer were determined on sections taken 
from the heads of the specimens. * 

Table i contains a summary of the results of the tests of all 
specimens as well as of the chemical compositions, and pom-ing 
and melting temperatures, the data of heat treatment, and other 
information. 

IV. INFLUENCE OF FORM OF TEST SPECIMEN 

In Figs. I and 2 are shown sketches of the forms of test bars used 
in these experiments. The form / is often used for metals of high 
shrinkage, such as steel or manganese bronze. The test piece is 
taken from one of the lower legs and may be machined to any de- 
sired form ; in these tests the form II A was used, the test specimens 
being tested with wedge grips. The form // has been accepted as 
standard by the American Society for Testing Materials in its speci- 
fication B-ro-i8 for 88:10:2 bronze; the dimensions of the bar used 
in these experiments do not correspond exactly to those of this 
specification, however. The bar may be machined for use with 
wedge grips or with threaded holders ; in these experiments all bars 
cast into this form were tested with wedge grips. Forms /// and 
VII are ordinary types of test bars from which a standard test 
specimen of 0.505 inch diameter may be obtained to be tested with 
wedge grips. The No. IV is given in the tables to a bar cast to the 
same dimensions as No. Ill, but machined vv^ith threaded ends. 
Bars V and VI are of the shoulder type, and are machined to 0.505 
inch diameter. Bars ///, IV, V, VI, and VII were cast in pairs, 
horizontally, in green sand, with gate and riser for each pair. 

The test results show that No. I gives very low values of the 
tensile strength, owing to the large section of the cast piece. This 
type is not satisfactory for aluminmn alloys. In all of the tests 



Technologic Papers of the Bureau of Standards 



P-iV-i— ft — rii-n 







1 


oz 


sk- 


A 






f 




— ^ _ 




■ 


* '. 






r 




0.57 


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Fig. 1. — Forms of test bars used 

I, a type often used for metals of high shrinkage such as for steel or manganese bronze. 

II, the t>-pe recommended by the American Society for Testing Materials for 83 : lo : 2 bronze; in the 
tests described herein the cast bars were machined to the form IIA and tested ■with wedge grips. 

///, a common tj'pe of test bar, poured horizontally in green sand, with gate and riser; the specimen is 
machined to the form IIA and tested with wedge grips. 

IV; this number is given in the tables to the same cast bar as III but machined with threaded ends and 
tested in a self-centering holder. 



Tests of Light Casting Alloys 7 

in which a direct comparison was made between bars of types // 
and /// the bars of type /// gave higher values both of the tensile 
strength and of the elongation than those of type //. (See heats 
E7, Table 16; Gi and G2, Table ic; Hi and H2, Table id.) 

The tensile strength obtained from bars of type /// was always 
less than that from bars of types V, VI, and VII on the same alloy. 
(See heats E9, Table \b; E23, E24, and E27, Table le.) This is 




2^ 



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- 1 

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Fig. 2. — Form of test bars used 

V; shoulder type of test bar; it is machined to 0.505 inch diameter over 2^ inches. 

VI; shoulder type of test bar; it is machined to 0.505 inch over i^i inches. 

VII; small bar similar to ///; it is machined to 0.505 inch diameter and tested with wedge grips. 

All of these bars are cast horizontal in green sand with gate and riser. 

undoubtedly due to the difference of section of the cast specimen; 
the bar III is cast with a diameter of 11/16 inch over the gage 
length, whereas the others are cast with one of 9/16 inch. The 
fact that such a small difference of section of the cast bar makes 
such an appreciable difference in the tensile strength shows the 
necessity for precise statement of this section in describing test bars 



8 Technologic Papers of the Bureau of Standards 

in specifications for light alloys. The diameter of 9/16 inch, used 
for many of these test bars, would seem to be a most satisfactory 
one, as it provides amply, but with a minimum amount of machin- 
ing, for obtaining the 0.505 inch diameter specimen from it. 

It was expected that direct comparison between the types III 
and VII, tested with wedge grips, and V and VI, gripped at the 
shotdder and self centering, would indicate the advantage of the 
latter type. It is generally conceded that wedge grips should not 
be used for aluminum alloys which are brittle if the best and most 
uniform results are to be obtained. Indeed, many instances of 
variation of tensile properties of test bars of type /// from the same 
heat were ascribed to the fact that they were not properly cen- 
tered in the testing machine. The results of tests of bars of heat 
E9, Table \h, show that, in this case at least, the bars tested with 
wedge grips {VII) gave as good results as those of the shoulder 
type (F and VI). It is to be observed that the effect of the use 
of wedge grips is much influenced by the carefulness of the oper- 
ator and the accuracy of alignment of the testing machine; the 
possibility of obtaining low results is always existent when they 
are used. 

V. INFLUENCE OF MELTING AND POURING 
TEMPERATURES 

The influence of poiuriag temperatiu'e on the mechanical prop- 
erties of light alloys has been the subject of much study. (See foot- 
notes I and 2.) As the pomring temperature is increased the tensile 
strength of the alloy decreases, due largely to its larger grain size. 
This is illustrated in the results of the tests of heat Gi-C-P, 
Table ic, and of E27, Table le. 

The effect of the maximum temperature of melting as distinct 
from that of pouring has been given apparently much less attention. 
Two tests were made to ascertain the effect of maximum tempera- 
ture of melting, using the same pouring temperature. The pot of 
metal was melted, allowed to cool to 700° C, and test bars cast at 
this temperature. The pot was then replaced in the furnace and 
heated to a higher temperature, removed from the furnace, and 
bars cast at this higher temperature; the pot was then allowed to 
cool again to 700° C and the remainder of the metal poured into 
bars at this temperature. Heats Gi , Table ic, and E27, Table le, 
were poured in this manner. The difference between the bars 

2 H. W. Gillett, The Influence of Pouring Temperature on Aluminum Alloys, Eighth International 
Congress of Applied Chemistry, Sec. II, 2, p. 105; 1912. 



Tests of Light Casting Alloys 9 

heated during melting to 850° and those to 790° C in the case of 
Gi, both being poured at 700° C, was not noticeable. In the case, 
however, of heat E27 a falling off in tensile strength is noticeable 
after heating at the higher temperature, 950° C. 

The two tests, while too few to be conclusive, seem to confirm 
the current belief that the maximum temperature during melting 
must not be allowed to go too high, probably not over 850° C. 

VI. INFLUENCE OF CHEMICAL COMPOSITION 

The subject of the effect of chemical composition on the mechani- 
cal properties of light aluminum alloy castings has been much 
investigated. (See footnotes 1,3, and 4.) 

The metals which are principally used to impart hardness and 
strength to aluminum are copper, zinc, and magnesium. Each 
of these is soluble to a Hmited extent in aluminum, and it is to the 
increased hardness of these several soHd solutions that the value 
of these additions is due. The present tests have been largely of 
alloys with copper as the principal hardening component. In 
this country the use of an alloy containing approximately 8 per 
cent of copper is very general; In fact the majority of sand castings 
at least are made of this composition. It is an excellent alloy for 
general purposes, hard and readily machined. Its only disad- 
vantage is its brittleness; test bars will average only about 2 per 
cent elongation in 2 inches. Castings made of it will resist wear 
well, and will not break under stresses which are below the yield 
point; they will not, however, withstand appreciable deformation 
without fracturing. It would seem that for many purposes an 
alloy might be desired which would be tougher and less fragile, so 
that instead of fractiu-ing under accidental overload, it would 
yield slightly without failture. 

A number of different compositions were tested with the purpose 
of ascertaining what tensile properties could be obtained with an 
alloy having appreciable ductility. In choosing the compositions 
for the alloys tested, the maximum solubility of copper in alu- 
minum was used as a guide; this is approximately 4 per cent. 
Beyond this amount the excess copper is present in the form of a 
brittle constituent which can not markedly increase the hardness 
but does decrease the ductility. 

'H. C. H. Carpenter and C. H. Edwards, Alloys of Aluminum and Copper, EiKhtli Report to Alloys 
Research Committee, Proc. Inst. Mech. Eng,; 1907. 

* W. Rosemliaim and L. S. Archbutt , Alloys ol Aluminum aud Zinc, Tenth Report to the Alloys Research 
Committee. Proc. Inst. Mech Eng.; 19:3. 

131039°— 19 2 



lo Technologic Papers of the Bureau of Standards 

The test results of alloys G, Table ic, of alloys H, Table id, and 
with E23, E24, E26, and K27, Table le, indicate the range of 
tensile properties which may be expected in a ductile alloy. It 
appears that there should be no difficulty in obtaining with a 
number of compositions the following tensile properties: 

Tensile strength lbs. per sq. in . . 20 000-25 000 

Elongation in 2 inches per cent. . 5 

It is to be observed that such alloys will not be as hard as the 
well-known one containing 8 per cent copper, and they will have 
a slightly higher shrinkage ; low shrinkage and high hardness must 
be sacrificed in order to obtain the higher ductility. 

One of the most striking facts apparent from a consideration of 
the mechanical tests of different compositions of light alloys is 
the hardening and embrittling effect of small additions of mag- 
nesium to alloys containing copper. This is evident from a com- 
parison of heats C15, C16, C17, and E27, all of which contain 
from 3 to 3.5 per cent of copper, the first three containing in 
addition from 0.4 to 1.3 per cent magnesium. With this copper 
content the elongation is lowered from about 8 to 2.5 per cent by 
the addition of magnesium, with only a slight increase in tensile 
strength. This fact is well illustrated also in heats Gy and 8 and 
H 5 and 6. In each of these cases the metal was melted and bars 
poured without magnesium; thereupon the magnesium was added 
to the pot and additional bars poiued. The tensile strength of 
each composition was increased by 10 to 20 per cent, the elonga- 
tion was decreased by 60 to 80 per cent by the addition of the 
magnesium. It seems apparent that high ductiUty can not be 
obtained in cast alloys containing magnesium. 

The results of the tests of heats E23, E24, E26, and E27, Table 
le, containing only copper, and of heats G, Table ic, and H, Table id, 
containing nickel and manganese in addition to copper, would indi- 
cate that tensile strengths as high as 25 000 pounds per square 
inch can not be obtained with the use of copper alone, but that 
nickel or manganese must also be added. On the other hand, if a 
tensile strength of 1 8 000 pounds per square inch is sufficient, it 
appears somewhat questionable whether better results are not 
obtained with copper alone; the addition of manganese seems to 
produce more tmiform test results, but the resulting alloys are 
somewhat less ductile than those with copper alone. 

Two heats of commercial alttminum were cast into test bars 
without the addition of hardener of any sort. The test results 
are given in Table la. 



■^ 



Tests of Light Casting Alloys n 

At one time there were two lots of notch-bar ingots in the 
foundry which seemed to show a marked difference in brittleness 
as tested quite roughly by bending the ingot. Inasmuch as the 
question of "brittle ingots" was a quite live one at the time, a 
heat of test bars of each lot was poured under the same conditions 
in order to ascertain whether the brittleness as so indicated would 
persist in the cast test bars. It will be noticed that there is a 
marked difference in the ductility of the bars of the two heats; 
the bars of the lot A-4 averaged only about 1 5 per cent elongation 
as compared with 29 per cent for lot A-2. It was, however, lot 
A-2 of which several ingots had been found to be brittle.^ It is 
interesting to note the difference in silicon content of the two 
heats; the one with the high silicon content gave the low values 
of ductility. 

VII. EFFECT OF HEAT TREATMENT 

It is well known that alloys containing copper with or without 
magnesium, in the forged or rolled condition may be improved by 
heat treatment consisting of quenching from about 500° C and 
allowing to age for several days. (See footnotes i, 6, and 7.) 
A number of tests were carried out to determine to what extent 
the mechanical properties of cast alloys might be improved by a 
similar treatment. Since it would not be desirable in many cases 
actually to quench castings in water, owing to the possibility of 
their cracking, the heat treatment which was applied in all of these 
tests consisted of heating at 500° C for two hours followed by 
cooling in air; the specimens were then allowed to age several 
days before testing. Such a heat treatment could readily be 
applied to most commercial castings of alimiinum alloy. In the 
tables the specimens which were so treated are marked "ac" 
or "fc" in column 10, depending on whether they were cooled in 
air or in the furnace after annealing. 

Test specimens of about 30 heats were so treated. In all but 
five or six cases there was found to be an increase in tensile strength 
resulting from this treatment amomiting to from 5 to 50 per cent 
of the strength of the cast, unheat-treated bars. In those cases 
in which the heat treatment resulted in a decrease of strength the 
whole group of bars of the heat was of inferior quality, cast and 
heat-treated ones alike. 

' It must be observed that not all iriKOts of the lot A-3 were brittle, and that there was no assurance that 
all of the ingots were actually oriiiinally of the same melt except that they belonged to the same purchase 
lot. 

" Wilm, Metallurgies, p. 650; 1911. 

'Cohn, Elcktrotcclmik u. MaschinenRau, 81, p. 430; 1913. 



12 Technologic Papers of the Bureau of Standards 

The effect of heat treatment on the elongation was more erratic ; 
in the majority of cases the average elongation of the heat-treated 
bars was lower than that of the unheat-treated ones, but in several 
cases there resulted an increase in ductility. Thus in the case of 
heats G7, Table ic, and H5, Table id, the elongation of the treated 
bars was approximately 20 per cent less than that of the unheat- 
treated ones, while in the case of heat E27, Table le, the elongation 
of the treated bars was approximately 50 per cent greater. 

The presence of magnesium seems in general to increase the 
amount of the hardening produced by heat treatment, at least 
in the amounts used in these tests. This is illustrated in the results 
of the tests of heats G7 and 8, Table ic, and of H5 and 6, Table id. 
This is quite in accord with the fact that the presence of magnesium 
in duralumin increases the amount of hardening produced by heat 
treatment.* 

It was observed that during the annealing of the specimens at 
500° C those which contained magnesiimi became covered with a 
dark gray coating of oxide, whereas those containing no magnesium 
remained quite bright and imaltered superficially during this 
treatment. 

It would seem that the application to light alrmiinum castings 
of heat treatment of a type similar to that described above has 
commercial possibilities, inasmuch as it is possible in this manner 
to obtain high values of tensile strength and hardness in alloys 
containing only from 3 to 4 per cent of hardener, which have a 
considerable ductility. 

VIII. RELATION BETWEEN TENSILE PROPERTIES AND 

HARDNESS 

The structure of cast almninum alloys is discussed below, and 
it is shown that these alloys fracture in most cases at the grain 
boundaries, the strength depending thus on the cohesion at these 
boimdaries. The hardness of the alloys depends principally, 
however, on the hardness or cohesion of the grains themselves. 
It is therefore not stuprising that the tensile strength and the 
hardness of cast light alloys do not bear any simple relation to 
each other. 

Thus the alloys of Table ih, containing about 8 per cent of 
copper, have an average Brinell hardness of about 65, a scleroscope 
hardness of about 20, together mth an average tensile strength 

8 Merica,Walteiiberg and Scott, The Heat Treatment and Constitution of Duralumin, Sci. Paper No. 337 
of the Bureau of Standards; 1919. Also in Bulletin Amer. Inst. Mech. Eng., No. 131, p. 1031; 1919. 



mt 



Tests of Light Casting Alloys 13 

of about 19 000 pounds per square inch. The alloys of the Gi and 
G2 series, Table ic, have about the same average tensile strength, 
but their Brinell hardness is only about 50, scleroscope hardness 
about II. Again the tensile strength of the bars of heat H5, 
Table id, is greater than the average tensile strength of the 8 
per cent copper alloy, but their hardness is less. 

The effect of heat treatment is to increase both the tensile 
strength and the hardness, although not always in the same pro- 
portion. Thus, in Table le, the tensile strength and hardness 
of the specimens of C56 were increased in about equal proportions, 
the tensile strength of the bars of C20 was increased in greater 
proportion than the hardness, while in the case of C44 the effect 
of heat treatment was to decrease the tensile strength and to 
increase the hardness. 

IX. MICROSTRUCTURES 

Photomicrographs illustrating typical microstructures of alu- 
minum light alloys are shown in Figs. 3 to 16; all of the alloys of 
which the structures are shown contain copper as the principal 
hardening element. Elsewhere ^ it has been shown that at 525° 
C approximately 4 per cent of copper is dissolved in solid solution 
in aluminum as CuAlj; the solubility of this compound in alumi- 
num diminishes at lower temperatures, and is about i per cent 
at 300° C. Cast alloys containing copper therefore have a struc- 
ture consisting of grains of aluminum solid solution of cored or 
dendritic arrangement, surroimded partially or completely by 
envelopes of the excess CuAlj. This is illustrated in the figures. 

The elements nickel and manganese also form compotmds with 
aluminum which are only slightly soluble in aluminum and form 
envelopes in the same manner as does CuAlj. The greater the 
sum of the percentages of copper, nickel, and manganese the 
greater the volume of the compounds formed and the more com- 
plete and continuous the grain envelopes. Compare, for example, 
the structure of E7-M11, Fig. 6, containing 8 per cent of copper 
with that of Gi-I, Fig. 7, containing 2 per cent of copper plus i 
per cent of manganese. 

The path of fracture in cast alloys having this structure when- 
ever possible follows these envelopes which are hard and brittle. 
This is shown in Figs. 14, 15, and 16, photomicrographs taken 

" Merica, Waltenbcrg, and Freernan, The MctalloRrnphy of Alujniniun and Its Lik'lit Alloys with Copper 
and with Magnesium, Sci. Paper No. 337 of the Bureau of Standards; 1919. Also in BullctiuAmcr. Inst. 
Mech. Kng. No. 151, p. 1031; 1919. 



14 Technologic Papers of the Bureau of Standards 

near or at the fractured ends of tensile-test specimens. When 
the envelopes are not complete and continuous, the fracture is 
forced in places to cross the grains. In Fig. 1 6 it is noted that 
the fracttire occurs both at the envelopes and across the grains. 

Besides increasing the volume of the envelopes of CuAl,, the 
increase of copper content causes an increase in the average con- 
centration of the solid solution CuAij in alvuninum; the hardness 
of the alloy depends principally on the hardness of the grains, 
which is increased roughly in proportion to the concentration of 
the solid solution. Increases of copper content therefore in- 
creases the hardness of the cast alloy, and up to a certain amount, 
about 3 per cent, also increases the tensile strength. Beyond that 
amotmt, although the hardness continues to increase, the increase 
in volume of the envelopes produces a more favorable path of 
ruptiu-e imder tensile stress, and the tensile strength is not further 
increased. 

The ductility of the alloys depends on the relation between the 
tensile strength and the stress necessary to produce permanent 
deformation. If the former appreciably exceeds the latter the 
alloy will exhibit ductility, otherwise it will not. Within a 
structirre such as that shown in Fig. 6, consisting of grains com- 
pletely surrounded by the brittle envelopes, fracture at the envel- 
opes occurs before the stress can be increased suE&ciently to pro- 
duce deformation. In a structure such as that of Figs, g and lo, 
the envelopes are broken up and discontinuous; the path of frac- 
ture is forced at least partially across the grains, and deformation 
within the grains can be produced before fracture occurs. The 
aUoy of Fig. 6 exhibits practically no ductility; those of Figs. 9 
and 10 from 4 to 6 per cent elongation. 

X. RESISTANCE TO CORROSION IN THE SALT-SPRAY TEST 

The resistance of these alloys to corrosion was determined by 
use of the salt-spray test. This test consists of exposing the 
samples to a continuous fog of salt water, produced by atomizing 
a 20 per cent solution of salt (sodium chloride) in water. ^^ 
Although this test is not considered as entirely satisfactor}^, it is 
thought that the results produced represent with a fair degree of 
accirracy the results obtained in actual service, especially imder 
marine conditions. 

1" A. N. Finn, Method of Making Salt-Spray Test. Proceedings Am. Soc. Test Mats., XYIII, Part I, 
p. 237; 1918. 



'mmm 



Bureau of Standards Technologic Paper No. 139 




Fig. 3. — specimen E$-M4. X20 



Fig. 4. — Specimen E^-AI^. Xioo 



1 




1 


^s 


^^^^^m 


m 





PM 


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^^^^^^^^^^H 




^^^^^^/.iM 




^^^^^^^^^3 




■K^^^^^vv^K^^^^^K^^^^S^sl^raf^ 




^^w^^^^l^B 


^^^^if^^^mi^^^^m-'^^mMmmms 



Fig. 5. — Specimen E6-M/. X50 



P'lG. 0. — Spi-cmiun L-]~Mii. X50 



The structure of the alloy containing 8 per cent of copper. All specimens etched with o.i per 

cent NaOH 



Bureau of Standards Technologic Paper No. 139 





Fig. 7. — Specimen Gi-1. X50 



Fig. 8. — Specimen Gi-D. X50 





Fig. 9. — Specimen H1-2. X50 Fig. 10. — Specimen A2-1. Xjo 

Structure of cast alloys. All specimens etched with o. i per cent NaOH 



^ 



Bureau of Standards Technologic Paper No. 139 















-V W- 



^A '-'^/-.r 









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4 



I '*f-V_,^ vv j^ 






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Fig. II. — Specimen H6-C. Xioo 




Fig. 12. — Specimen Clj-B. Xjo 




Fig. i;^.'- Specimen Cig-C. X30 



Bureau of Standards Technologic Paper No. 139 




Fig. 14. — Specimen p7 E. X150 




Fig. 15. — Specimen E5-M1. X50 
The path of fracture in cast alloys 



I^M 



Bureau of Standards Technologic Paper No. 139 




Fig. i6. — Specimen H2-4. X50 

The path of fracture in cast alloys 



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Tests of Light Casting Alloys 



15 



The alloys were subjected to the salt-spray test for two periods 
of one month each, and were examined at the end of each period 
to determine the relative amount of corrosion. This was esti- 
mated by appearance only, as it is practically impossible to deter- 
mine it by loss in weight on account of the adherence of the 
products of corrosion and the lack of a satisfactory reagent to 
remove the rust without affecting the metallic aluminmn. 

The alloys were cast 1/4 inch thick, about 1/8 inch was then 
milled from one surface so that one exposed surface was as cast, 
and the other was machined. The specimens were 2 inches 
wide and 4 inches long. 

The designation of the cast alloys, with their composition and 
treatment, is given in the following table: 



Marked 


Treatment 


Composition 


A-30-A 


As cast 


Al+5 per cent Mg. 


A-30-A-A 


Annealed 500° C 


Do. 


CA 40 A .... 




Al+3 per cent Cu+1 per cent Mg. 
Do. 


CA 40 A A 


Annealed 500° C 


GIF 


As cast 


Al+2 per cent Cu+1 per cent Mn. 
Do. 


G 1 F A 


Annealed 500° C 


Z 31 A 


As cast . . 


A1+13J per cent Zn+3 per cent Cu. 
Do. 


Z 31 A A 


Annealed 500° C 


E A 




Al+8 per cent Cu. 
Do. 


E A A 


Annealed 500° C 









These cast alloys appeared very slightly and about equally 
corroded after two months exposure to the salt spray, although 
the alloy containing 5 per cent Mg (A-30-A, and A-^30 A- A) 
appeared somewhat better than the others. No difference could 
be detected between the milled surface and the surface as cast, or 
between the specimen as cast and annealed. 

The authors are indebted to A. N. Fimi for his assistance in 
carrying out the salt-spray tests. 

XL RESISTANCE TO THE ACTION OF ALTERNATING 

STRESSES 

Light aluminum casting alloys are called upon not only to 
resist the action of a constant load or stress but also frequently 
to resist vibratory stresses, as for example, in a crank case. 
There is little published information on the subject of the resist- 
ance of light alloys to the action of alternating stresses. Elmen- 
dorf" finds that cast commercial aluminum, subjected to the 



" A. Elmendorf. Tables and Charts Rcsultini; from the Testing of Cast Aluminum on a Wliite-Southei 
Rotary Testing Machine, Amcr. Mach., 41, p. 8it; 19:4. 



i6 



Technologic Papers of the Bureau of Standards 



White-Souther test will withstand i ooo ooo reversals of stress 
at a maximum fiber stress of lo ooo pounds per square inch. 
Jeffries ^ states that the cast alloy containing 8 per cent of copper 
will withstand an unlimited number of alternations of tensile 
stress between o and 12 000 poimds per square inch. 

Three compositions of alloy were tested in this respect: 
(i) That containing 8 per cent of copper (the E series) ; (2) That 
containing from 2 to 3 per cent copper and from 12 to 15 per 
cent zinc (the Z series) ; and (3) that containing from 1.5 to 2 per 
cent copper and from 1.5 to 2 per cent manganese (the G series). 
The test bars were cast horizontal in green sand, with gate and 
riser, each being 27 inches long and 7/8 inch in diameter. The 
bars were machined for test to a tmiform diameter of 0.740 inch. 

A modification of the old Wohler or White-Souther machine 
was used for the testing. Each bar was supported at the two 
outside points and the load applied at the two intermediate 
third points by means of weights in the usual manner. These 
points were 8 inches apart. The specimens were then rotated at 
an approximately imiform rate of 1000 rpm, the number of 
rotations to rupture being indicated in the usual manner with a 
revolution counter. 

The results of the tests are given in Table 8 and in the curves 
of Fig. 17. For comparison the tensile properties determined on 
the usual type of tensile test bar, of the heats from which alter- 
nating stress test bars were poured and tested, may be seen in 
Tables i&, ic, and if. 

There seems to be no marked difference between the different 
compositions as regards their resistance to alternating stresses. 
The series E seems to be somewhat superior to the others, the 
series G, slightly inferior. The curves in Fig. 17 are drawn 
through the mean of the mean number of alternations to ruptture 
for each stress. The following values are taken from the three 
curves: 



Maximum fiber stress withstood at 1 000 000 alternations . . 
Maximum fiber stress withstood at 10 000 000 alternations. 
Maximum fiber stress withstood at 100 000 000 alternations 



Series 
E 



Series 
Z 



11 000 
8200 
6000 



10 400 
7200 
5000 



Series 
G 



9500 
6900 
5000 






Tests of Light Casting Alloys 



17 







i" p. a 



6 



1 8 Technologic Papers of the Bureau of Standards 

These results are apparently best expressed in the fonn of 
curves or in such a table, but the following equations of the usual 
type are calculated from the mean curves: 

For the E series S = 68 coo N~^-'^^' 
For the Z series S = 93 000 A' ~°-^^* 
For the G series S = 65 000 A'-^-^^^ 

When 5 = the maximum fiber stress in pounds per square inch, 
N = number of alternations to ruptiu*e. 

These equations may be compared with that given (loc. cit.) 
by Ehnendorf : 

5 = 48 000 A^-o-"3 

for cast comnaercial aluminum. It appears that the resistance to 
the action of alternating stresses does not increase in proportion 
as the tensile strength or the hardness of the alloy increases. 
Thus according to Ehnendorf aluminum alone will withstand as 
many alternations of stress at fiber stresses of from 7000 to 10 000 
poimds per square inch as will the alloy G, which has a tensile 
strength from 20 to 50 per cent higher than that of the aluminiun. 
The authors are indebted to L. J. Larson for his assistance in 
carrying out the above tests. 

XII. SUMMARY AND CONCLUSIONS 

The tensile properties and the hardness of a number of different 
compositions of light aluminum casting alloys have been deter- 
mined; the resistance to corrosion compared and the resistance to 
the action of alternating or vibratory stresses determined of a few 
commonly used compositions. 

It is advisable to use for tensile tests a test bar cast almost to 
size; a bar cast with the test length 9/16 inch in diameter, 
afterwards machined to 0.505 inch gives satisfactory results. 
The use of a type of test specimen which can be gripped in a self- 
centering holder in the testing machine is recommended. 

A study of the effect of chemical composition on the mechanical 
properties has shown that it is possible to obtain an alloy contain- 
ing from 2 to 3 per cent of copper together with i or 2 per cent 
of nickel, manganese, or both, which will have a reasonable 
amount of ductility, and it is believed that an alloy of this type 
should have commercial value. Tensile properties suggested for 
such an alloy are the following: 

Tensile strength pounds per square inch . . 20 000-25 000 

Elongation in 2 inches, not less than per cent . . 5 



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Tests of Light Casting Alloys 19 

The addition of magnesium to alloys containing copper reduces 
in a marked manner the ductility, but increases the tensile strength 
and the hardness. 

The effect of heat treatment on test-bar castings, consisting of 
annealing at 500° C, cooling in air from this temperature and allow- 
ing to age for several days, is to increase the tensile strength and 
the hardness; the ductility of the alloy is generally decreased, 
but may in some cases be increased. The presence of magnesium 
in the alloy in amoxmts of from 0.5 to 1.5 per cent seems to in- 
crease the hardening effect of heat treatment. The heat treat- 
ment of light aluminum castings would seem to have commercial 
possibilities. 

The microstructure of the different alloys was studied and it 
was found that fractm-e in them prefers a path along the brittle 
envelopes surrounding the grains of aluminum, consisting of the 
various eutectics which are formed with the added m.etals or their 
compoimds with altmiinum. 

Two months' exposure in the salt spray produced only slight 
corrosion of several compositions of cast alloys. There was no 
very appreciable difference between the different compositions in 
resistance to corrosion. 

A study of the resistance to the action of alternating stresses 
of three compositions of light cast alloys {E series, containing 8 
percent copper; Z series, containing 2 to 3 per cent copper, and 
12 to 15 per cent zinc; G series, containing 1.5 to 2 per cent copper, 
and 1.5 to 2 per cent manganese) showed that there was no 
marked difference in the behavior of the three alloys in this 
test, although the E series was somewhat superior and the G 
series somewhat inferior to the others. All of the three alloys 
will withstand 10 000 000 complete reversals (tension to com- 
pression) at a maximum fiber stress of 7000 pounds per square 
inch. 

The authors are indebted to Miss H. C. Baker and Col. A. C. 
Krynitzky for carrying out most of the mechanical tests. 



20 



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